In connection with measuring physical parameters such as pressure and temperature, it is previously known to utilise various sensor systems by which the optical intensity of a ray of light, conveyed through an optical fibre and coming in towards a sensor element, is influenced due to changes in the respective physical parameter. Such a system may for example be used when measuring the blood pressure in the veins of the human body. Said system is based upon a transformation from pressure to a mechanical movement, which in turn is transformed into an optical intensity, conveyed by an optical fibre, which is in turn transformed into an electrical signal that is related to the measured pressure.
According to known art, such a fibre-optical measurement system may comprise a pressure sensor, an optical fibre connected to said pressure sensor, and at least one light source and at least one light detector located at the opposite end of the fibre, in order to provide the pressure sensor with light, and to detect the information-carrying light signal returning from the pressure sensor, respectively.
One problem occurring with previously known systems of the above kind relates to the fact that the detected signal will be influenced by various interference factors in connection with the measuring system. For example, the signal may be influenced by any bending of the optical fibre, and by temperature changes and ageing of the optical fibre or of said light source. Also factors such as fibre couplings and fibre connectors in the measuring system in question may influence the information-carrying signal (for example through influencing its intensity in an unwanted manner) and thus also the final measuring result.
As a result of the above problems there is a need for devices and methods arranged for compensation of any existing sources of error and interference in connection with optical measurements of for example pressure.
There are several previously known measuring systems in which a measuring signal is used together with a separate reference signal. A certain measuring system category is based upon conveying light through two different optical fibres, and is used for said purpose. One example of such a system is shown in the patent document U.S. Pat. No. 5,657,405, which describes a fibre-optical measuring system used for measuring of e.g. pressure. In this system, the interference occurring between two optical conduits through which two corresponding laser light signals are directed towards a membrane, is utilised. One of these light signals is hereby used as a reference signal.
Another previously known category of systems is based on generating and utilising light of two different wavelengths, whereby a reference signal may be obtained. Systems of this kind are previously known from for example the patent documents U.S. Pat. No. 5,280,173 and U.S. Pat. No. 4,933,545.
One disadvantage with the systems according to the two categories mentioned above is that they are relatively complex in their structure. They further require a relatively large number of critical components in the form of LED:s, optical fibres, etc.